"However, they were being investigated. But if you look at his spreadsheet from a criminal history perspective, it remains unremarkable."

He added that Ney was listed by his family as a missing person.

"They had concerns for his welfare," he said. "Police had taken a report for that, and a report around a domestic violence situation. He was on the system as 'keep a look out for him' for both of those things, for both as a missing person.

"So from our perspective, again, that is not unusual in terms of what we see day in and day out in some houses across New South Wales.

"And that is not a common theme for someone to then take the next step of coming into the streets of Sydney with a knife and killing people and threatening to kill people.

"I feel as though what he has on social media, what we may well find in his bedroom, will hopefully shine more light on why this crime happened.

"At the same time, we will try to interview him as soon as he's released from hospital, which will hopefully again shine more light on what his intent was."

Ney is now being treated at the Royal Prince Alfred Hospital, where he is being guarded by police.

WHO IS THE ACCUSED SYDNEY STABBER?

Ney, 20, grew up in a weatherboard home in Marayong, a suburb near Blacktown in Sydney's west.

It was in Blacktown where Mr Fuller says Ney "self-admitted himself" to hospital earlier this month following a medical issue and there was "subsequent mental health assessments".

Ney was the middle of three siblings and the son of Turkish-Cypriot parents, and attended Marayong Public School followed by Blacktown Boys High.

Peers who went to school with the alleged stabber described him as a lonely child who didn't seem to have many friends.

"He was not much of a talker," Reen Elomari, who went to primary school with Ney, told the Daily Mail. "He always used to walk fast, not many friends, never used to say anything. I always used to see him roam around the streets alone. I would see him around my house. I'd see him maybe a couple of times … just alone."

A former high school classmate told the outlet the stabber's social media activity had included posts about converting to Islam.

"I always thought he was a bit weird and he was always posting (on Facebook) about converting and stuff like that," the classmate said. "I didn't expect him to go this far."

Police spent the night at Ney's Marayong home, which he lived in with his mother and sister. A neighbour, Joel, told Nine News he didn't often interact with other residents.

"Quiet guy, keeps to himself," he said. "I hit him up about something last week and (there was) only a few words said."

The man was restrained by members of the public with a milk crate and chairs. Picture: Channel 7

Investigators do not believe the Sydney incident was terrorism-related and said he did not have links to any terrorist organisations.

Mr Fuller said they found information on the man "suggesting he had some ideologies related to terrorism".

However, Mr Fuller said this is not evidence to call yesterday's incident an act of terrorism.

"Just having some footage saved on a USB drive is not a leap far enough for me to say that this is a terrorist incident," he said. "But obviously, it's extremely concerning and it is the starting point of a long-term investigation.

"I think he's a criminal. And I think that he came in with the intent to stab and injure people and that's my primary focus. The mental health consideration will, of course, occur through the potential judicial process.

"And I think, in anyone who commits an act of mass killings, how can they not be mentally ill, I suppose. But for mine, they're a criminal first and the issue around mental health is a distant second."

Ney was described as a lonely child who was socially awkward and didn’t have many friends. Picture: Kartik Lad/Twitter

The alleged attacker has a history of mental health problems, homelessness and drug use.

According to The Daily Telegraph, he was known to law officials, and just two months ago was let off by a magistrate after being caught with knuckledusters.

Police seized the weapons when they were called to his family home under a "concern for welfare".

He pleaded guilty to possessing a prohibited weapon without a permit and was sentenced to a conditional release order for nine months on conditions including that he does not commit another offence in that time. He was also instructed to seek mental health help.

Ney reportedly also had drug issues which he sought to deal with at Blacktown Hospital, and was spoken to by police over a domestic violence incident regarding his sister.

Police

The man is expected to be charged at Royal Prince Alfred Hospital today.